The invention relates to a selective overcurrent protection device, intended for AC operation; it is particularly suitable for protection of very high voltage systems (for instance the 400 KV voltage system of the French national grid system) against short circuits.
A disadvantage of existing protection devices is a possibility of actuation not only by a short circuit current but also by the intense magnetizing current surge under transitory conditions when large machines, particularly high power transformers, are switched on. Consequently, such existing devices may be triggered untimely at switch on of the transformers if a relatively low protection level is desired, that is to say corresponding to a low value of short circuit current. This untimely operation imposes on the transformers additional stresses because of a number of switch-ons greater than normal and cause premature wear of the protecting circuit breakers, due to the increased number of useless operations.
It is an object of the invention to provide a selective protection device, providing rapid discrimination between the two types of current likely to appear; it is a more specific object to make it possible to select a low protection level without a risk of numerous untimely tripping operations.
For that result, the invention uses the fact that the variation in time of magnetizing currents under transitory conditions is very different from the trend of the short circuit currents under steady and transitory operating conditions. As shown in FIG. 1, the curve representing the value of a magnetizing current under transitory conditions, as a function of time t, is formed of half waves of high amplitude separated by practically zero half waves of the opposite sign. The envelope of the peaks of the half waves decreases and tends towards zero when approaching permanent conditions.
On the contrary, the short circuit current under steady (established) conditions, (FIG. 2) is represented by a sine function. Consequently the same amplitude in absolute value occurs repeatedly at time intervals T/2 (T being the period i.e. 20 ms for a 50 Hz AC current).
The problem is however much less simple than appears at first sight, for the short circuit current may occur under transitory conditions. The maximum asymmetry corresponds to a variation of the current as a function of time of the type: ##EQU1##
The second term is significant only during an initial period of build-in. Refering to FIG. 3, it is seen that function becomes equivalent to that representing the fault current under established conditions as soon as t exceeds about 5.pi..
It could have been presumed that the possible existence of totally asymmetric operating conditions at cut-in would set aside a discrimination based on the difference of trends of the short circuit currents under permanent operating conditions and of the magnetizing current. A protection device in accordance with the invention, for satisfactory discrimination despite the possibility of such a situation, comprises a full wave rectifier, means for comparing the rectified current and a threshold value and for making a new comparison, if the rectified current exceeds the threshold, after a time equal to T/2+.DELTA.T (T being the period of the AC current and .DELTA.T a fraction of T of the order of one tenth) after the threshold was exceeded and means for tripping the device should the overshoot be renewed during the new comparison.
The invention will be better understood from the following description of a particular embodiment thereof, given by way of example only